Preservation ABCs: J is for Joist

Preservation ABCs is a series that will work its way from A to Z, bringing words into conversation that are relevant to historic preservation, whether it’s an idea, feature or vocabulary term. The idea is to help you see preservation everywhere you look and wherever you go. Enjoy! See previous letters.


J is for Joist


Historic preservation includes many fields and subjects, but at the core of the preservation field itself is buildings. Understanding the construction of a building and its elements will go a long way in conversation and in finding solutions. When looking at the “bones” of a building, joist is a good component to recognize. In the above picture the three members shown lengthwise are called joists.  A joist is a parallel horizontal beam that supports the floor and ceiling boards.

What you are looking at in this image are three joists with the floorboards of the second floor shown above the joists. In this particular instance, the ceiling plaster and lath has been removed, exposing the space between the ceiling and the second floor. The white marks on the joists are marks left behind from the plaster keys. You can also see knob and tube wiring (white ceramic cylinders and black wires) as well as new electrical wiring (yellow wire).  Joists can be made from wood (timber), steel, or reinforced concrete.

Joists are important, obviously, as they hold up ceilings and support floors. The best way to illustrate the function of joists? In college my professor told us that if we were unsure of the structural stability of a floor to be sure to walk on the joists. You’re more likely to go through a floor than a joist (not that you should put yourself in such a precarious position in the first place, of course).

The moral of this story? Learn your structural elements. And walk on the joists.

4 thoughts on “Preservation ABCs: J is for Joist

  1. jane says:

    not uncommon for carpenters to go through the ceiling when they also forget to walk on the joists. But old joists can have dry rot, powder post beetle damage or be cut and compromised – usually to allow for new plumbing or heating. So check from below before you walk up above.

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